The plerocercoid larva or spargana of Spirometra mansonoides (a species of tapeworm) is known to stimulate growth when implanted into several species of host animals This growth promoting activity is due to a factor elaborated by the spargana, which mimics the action of the vertebrate hormones, growth hormone (GH) and prolactin (PRL). There is some evidence that this factor is proteinaceous but it has not yet been isolated. The goal of this proposal is to isolate spargana growth factor (SGF) in highly purified form and to chemically, immunochemically, and biologically characterize the purified product. The properties of SGF will be compared with those of GH and PRL from a wide variety of vertebrate species. This comparison can yield valuable insights into the chemical relationship of SGF to the vertebrate hormones whose action it mimics, the mechanism of action of these hormones, and perhaps the structural features of GH and PRL necessary for biological activity. Purified SGF is also essential for studies on the possible role of SGF in the tapeworm itself, and its natural host species. These studies can provide information on the basic biological processes of growth and host/parasite relationships.